Assisting the older driver - SWOV

Intersection design and the older driver
Stop- and yield-controlled intersections
At stop‐ and yield‐controlled intersections having opposite left‐turn lanes
(and at intersections where protected‐only operations are not possible, e.g.,
because of an unacceptable reduction in capacity), Staplin et al. (2001)
suggest that safety could be improved by adjusting the left‐turn lane
geometry. Opposite left‐turn lanes and the traffic that uses these lanes can
restrict the left‐turning driver’s view of oncoming traffic in through lanes.
The level of blockage depends on how the opposite left‐turn lanes are
aligned with respect to each other, as well as the type and size of the vehicles
in the opposing queue. Restricted sight distance can be minimized or
eliminated by shifting opposite left‐turn lanes to the right (positive offset) so
that left‐turning drivers do not block each other’s view of oncoming traffic
(Staplin, Harkey, Lococo & Tarawneh, 1997). Figure 5.1 shows the difference
between opposite left‐turn lanes that are exactly aligned (i.e., no offset) and
the situation where the opposite left‐turn lane is shifted to the right (i.e.,
positive offset).
Figure 5.1. No offset versus positive offset (Staplin et al., 2001).
It is not so much the degree of positive offset that matters, as does the sight
distance that results from a certain positive offset. Left‐turn lanes frequently
used by heavy lorries need a larger offset, since these lorries have a more
adverse effect on sight distance. McCoy, Navarro and Witt (1992) have
developed an approach that can be used to compute the amount of offset that
is required to minimize or eliminate the sight restriction caused by opposing
left‐turn vehicles. Where the provision of unrestricted sight distance is not
feasible, Staplin et al. (2001) recommend to compute stopping sight distance
using a perception‐reaction time of 2.5 second to allow for the slower
information processing of older adults.
A possible negative side effect of a positive offset of canalized left‐turn lanes
(e.g., a parallel or tapered left‐turn lane between two medians) is the
potential for wrong‐way manoeuvres by drivers turning left from an
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